The fatigue strength of materials is of importance in attaining greater capacity and in improving reliability for a device under applied loads. This is particularly true with respect to aerospace components such as turbine blades, but many other examples exist as well. Most fatigue and stress corrosion failures originate at the surface of a part. In or near a surface, residual stresses are generated after plastic deformation that is caused by applied mechanical loads, thermal loads or phase changes. Residual stresses are known to affect the initiation and the growth of fatigue cracks. In general, tensile residual stresses are undesirable since they add to applied stress levels and lead to fracture at lower loads than might be expected. Compressive residual stresses in the surface of a part are beneficial because they act against applied loads and tend to increase fatigue strength and fatigue life, slow crack propagation, and increase resistance to stress corrosion cracking.
With that in mind, in order to enhance fatigue strength and resist stress corrosion cracking in materials, compressive residual stresses have been known to be introduced into surfaces of the materials. A number of methods for doing so are known, one category of which is referred to as peening. Peening is defined as the process of altering the surface of a material by impact. A variety of peening processes have been proposed and implemented in the past. For example, shot peening is accomplished with the use of air or centrifugal propelled shot aimed to impact the surface of the material part. The shot media may be solid round objects such as spherical cast steel shot, ceramic bead, glass bead or conditioned cut wire. After the shot peening process the surface finish of a shot peened surface is not ideal and often must be carefully machined to provide close dimensional tolerance. However, this machining is done at the risk of a loss in compressive stress depth. Other difficulties include contamination, process control, and waste disposal of used shot.
Another type of peening, known as cavitation peening, is performed by creating cavitation bubbles within a water jet beam near the surface to be treated. The shock of the collapsing bubbles causes water to strike the surface of the part, plastically deforms the surface, and leads to the formation of the sought-after compressive residual stresses. Often the material part to be treated is submerged under water while a water jet is directed at the surface of interest to create cavitation bubbles. In some cases, it is deemed essential to vibrate the nozzle of the water jets in order to induce cavities in the jet stream. The requirement to submerge the material part in water is costly, inconvenient, and often not possible.
One type of water jet peening places the material part to be treated in a water tank. A high pressurized water stream with hard, round media suspended therein impinges on the surface of the material part and introduces compressive residual stresses. The media remains in the water after the completion of the process but can be strained out. Since this process requires media, it leads to higher cost and if the media is to be reused, extra steps have to be added to remove the damaged media.
Moreover, all of the above-mentioned peening processes are considered a separate step in the manufacturing process flow. As a result, they add costly manufacturing time because of the additional steps of removing the material part from the manufacturing process, moving the material part to the peening process, applying peening to the surface of the material part, removing the material part from the peening process, and sending the material part to the next manufacturing process.
Still other processes, above and beyond peening, are also known to impart compressive residual stresses on fatigue critical parts. Such processes include cold rolling, low plasticity burnishing, roller burnishing, ultrasonic peening, and laser shock peening. However, as with the above, all these processes have their incumbent benefits and detriments. For example, they can result in surface conditioning, undesirable depth of compressive stress, machine complexity and capital cost. In addition, all these processes add an additional step to the manufacturing process. Some of the above mentioned limitations have been overcome and discussed in details in U.S. patents application Nos. 13/351,380 and 13/355,831.
It would therefore be beneficial if an improved peening process for introducing compressive residual stresses were to be developed. Moreover, it would be desirable for the new process to not require removing the material part to be peened from the manufacturing process.